1. Field of the Invention
The present invention relates to nonreducing dielectric ceramics and ceramic electronic components, such as monolithic ceramic capacitors and monolithic LC filters, using the nonreducing dielectric ceramics. More particularly, the invention relates to improvements in nonreducing dielectric ceramics so that cofiring with copper or copper alloys can be performed.
2. Description of the Related Art
Dielectric ceramics containing tungsten-bronze-type crystal phases, such as a BaRE2Ti4O12-type crystal phase and a BaRE2Ti5O14-type crystal phase, where RE is a rare-earth metallic element, which are used for ceramic electronic components such as temperature compensating type ceramic capacitors, are disclosed in Japanese Examined Patent Application Publication No. 55-20602, etc. Since the dielectric loss is small (the Q factor is high), they are suitable for use in the ceramic electronic components, and since the relative dielectric constant ∈ is relatively high at 70 to 80, the ceramic electronic components can be miniaturized.
On the other hand, ceramic electronic components which are used in the high-frequency band such as monolithic LC filters, must have internal conductors built therein with high conductivity. Therefore, highly conductive materials, such as gold, silver and copper, must be used as the conductive materials for the internal conductors. It must also be taken into consideration that metals contained in the internal conductors built into monolithic ceramic electronic components can be cofired with dielectric ceramics. Moreover, metals used for the internal conductors are desirably relatively inexpensive.
Because gold or silver used as the conductive material are relatively expensive, the cost of the resultant ceramic electronic component is increased. Also, silver has a low melting point of approximately 960xc2x0 C., and sintering of ceramics below this temperature often causes difficulties. Furthermore, conductors, such as internal electrodes, are formed in contact with a ceramic in monolithic ceramic electronic components, and the use of silver for the conductors may result in migration.
In contrast, since copper is relatively inexpensive, the cost of the resultant ceramic electronic component can be kept low. Since copper also has the highest melting point of approximately 1,080xc2x0 C. and has a high conductivity, it is suitable as the material for conductors built into ceramic electronic components used in the high frequency band. However, when copper is used as the conductive material in the fabrication process, firing must be performed in a neutral or reducing atmosphere. Therefore, the dielectric ceramic used in the monolithic ceramic electronic component must have a nonreducing property in addition to the high relative dielectric constant, high Q factor and high temperature stability.
A BaRE2Ti4O12-based or BaRE2Ti5O14-based tungsten-bronze-type crystal phase requires a high firing temperature of 1,300 to 1,400xc2x0 C. in order to perform sintering. It is therefore not possible to use copper as the material for a conductor which is cofired with these crystal phases.
Additionally, if the tungsten-bronze-type crystal phase is fired in a neutral or reducing atmosphere, it becomes semiconductive because it has a poor nonreducing property, resulting in degradation in insulation resistance and dielectric loss.
Furthermore, since the tungsten-bronze-type crystal phase has a high rate of change in capacitance with temperature of approximately xe2x88x92100 ppm/xc2x0 C., the applications of the ceramic electronic component containing this crystal phase is limited.
It is an object of the present invention to provide a nonreducing dielectric ceramic which can be cofired with a conductor composed of copper or a copper alloy as the conductive material, and in which the insulation resistance and the dielectric loss are not greatly degraded even if firing is performed in a neutral or reducing atmosphere, and the rate of change in capacitance with temperature is low, within xc2x160 ppm/xc2x0 C. It is another object of the present invention to provide a ceramic electronic component using the nonreducing dielectric ceramic.
In one aspect of the present invention, a nonreducing dielectric ceramic contains a tungsten-bronze-type crystal phase including at least Ba, RE and Ti as elements, and a pyrochlore-type crystal phase including at least RE and Ti as elements, where RE is at least one rare-earth element and the relationship 0.10xe2x89xa6b/(a+b)xe2x89xa60.90 is satisfied, where a is the maximum peak intensity assigned to the tungsten-bronze-type crystal phase and b is the maximum peak intensity assigned to the pyrochlore-type crystal phase determined by X-ray diffractometry.
Preferably, the nonreducing dielectric ceramic of the present invention contains a principal constituent including the tungsten-bronze-type crystal phase and the pyrochlore-type crystal phase, and contains, as secondary constituents, about 3 to 35 moles of Mn relative to 100 moles of Ti in the principal constituent, about 3 to 25 parts by weight of a glass component containing B2O3 relative to 100 parts by weight of the principal constituent, 0 to about 25 moles of Cu relative to 100 moles of Ti in the principal constituent, and 0 to about 25 moles of V relative to 100 moles of Ti in the principal constituent.
Preferably, the tungsten-bronze-type crystal phase is at least one of a BaNd2Ti4O12-type crystal phase and a BaNd2Ti5O14-type crystal phase in the nonreducing dielectric ceramic of the present invention.
Preferably, the pyrochlore-type crystal phase is an Nd2Ti2O7 type crystal phase.
The BaNd2Ti4O12-type crystal phase, the BaNd2Ti5O14-type crystal phase and the Nd2Ti2O7-type crystal phase are not limited to BaNd2Ti4O12, BaNd2Ti5O14, and Nd2Ti2O7, respectively, and, for example, Nd may be partially replaced with another rare-earth element.
In another aspect of the present invention, a ceramic electronic component includes an electronic component body including the nonreducing dielectric ceramic and a conductor in contact with the nonreducing dielectric ceramic. The electronic component body of the ceramic electronic component may have a multilayered structure in which a plurality of dielectric ceramic layers composed of the nonreducing dielectric ceramic are laminated or may have a so-called single-layer structure.
The nonreducing dielectric ceramic of the present invention can be cofired with copper or a copper alloy, and even if firing is performed in a neutral or reducing atmosphere, the insulation resistance and the dielectric loss are not greatly degraded, and it is possible to provide a low rate of change in capacitance with temperature within xc2x160 ppm/xc2x0 C.
Consequently, when such a nonreducing dielectric ceramic is used for an electronic component body in a ceramic electronic component including a conductor in contact with the electronic component body, it is possible to produce a ceramic electronic component which is highly reliable with respect to electrical characteristics, such as insulation resistance and dielectric loss, and which has superior temperature stability.
Preferably, the ceramic electronic component is a monolithic ceramic capacitor including a plurality of dielectric ceramic layers composed of the nonreducing dielectric ceramic and at least one pair of internal electrodes disposed with some of the dielectric ceramic layers therebetween.
The ceramic electronic component of the present invention can be a monolithic LC filter including a plurality of dielectric ceramic layers composed of the nonreducing dielectric ceramic and internal electrodes forming inductance L and capacitance C among the dielectric ceramic layers.
If the nonreducing dielectric ceramic is used as the ceramic constituting the dielectric ceramic layers, reliability in the electric characteristics, such as insulation resistance and dielectric loss, can be enhanced, and the rate of change in capacitance with temperature can be decreased. Consequently, the nonreducing dielectric ceramic of the present invention can be advantageously used as dielectric materials for temperature compensating type ceramic capacitors, microwave dielectric resonators, multilayered circuit components, etc.
In such a ceramic electronic component of the present invention, preferably, the conductor or the internal electrode includes elemental copper or a copper alloy as a conductive material. Since copper or a copper alloy can be used as the conductive material contained in the conductor or the internal electrode in the ceramic electronic component, the cost of the ceramic electronic component can be kept low, and also superior high frequency characteristics can be attained.